1. Field of the Invention
The present invention relates to a throttle actuator for use in an internal combustion engine and, more particularly, to a throttle actuator making use of a motor having a rotatable output shaft.
2. Description of the Prior Art
In the field of automotive internal combustion engines, there is a trend for realization of electronic control of throttle valve.
The electronic control of throttle valve generally employs a throttle actuator incorporating, for example, a stepping motor which is controlled in accordance with a throttle opening signal representing the instant degree of operating of the accelerator, thereby conducting feedback control of the opening of the throttle valve under various conditions. The electronic control of throttle valve is expected to provide various advantages such as higher fuel economy and cleaning of exhaust gases, through improvement in the precision of control of the throttle valve opening.
An electronic throttle control device of this kind is disclosed, for example, in Japanese Examined Patent Publication No. 58-25853. Another type of electronic throttle control device has also been known in which the opening of the throttle valve is controlled by means of a solenoid. An example of the electronic throttle control device of this type is shown, for example, in Japanese Unexamined Utility Model Publication No. 60-23238. In this device, the solenoid is cooled by engine coolant which is circulated through the engine.
In general, the temperature in the engine room is variable over a very wide range, from about -30.degree. C. up to 100.degree. C., which is experienced from the time that the engine is not operating while the automobile is used in a cold season to a time when the engine is operating in a high load while the automobile is used in a hot season. When a motor is used as the actuator for electronic throttle control system, the inside temperature of the motor rises to a level which is still higher than the temperature of air in the engine compartment, due to the heat of this air and the heat generated by the motor itself. When the air temperature in the engine compartment is high, therefore, it is often experienced that the throttle actuator motor is overheated to a level which exceeds the maximum allowable temperature. In such a case, the impedance of the coil in the motor is increased so that the output of the throttle actuator is reduced with the result that the throttle valve cannot be actuated properly.
The same problem is encountered also with the case where the electronic throttle control system employs a solenoid as the actuator. Namely, the solenoid tends to be overheated due to high temperature of air in the engine compartment so that the output power is decreased to impair correct movement of the throttle valve, and this is the reason why the solenoid is cooled by the engine cooling water as explained above. Another problem encountered with the electronic solenoid control system employing a solenoid-type actuator is that the control of intake air is possible only when the air flow rate is comparatively small due to restriction in the power of the solenoid, e.g., only when the engine is operating at a comparatively low speed of about 3,000 rpm or so. When the function of the actuator is impaired due to a rise in the temperature of the air in the engine compartment, the movement of the throttle valve is seriously affected by the intake vacuum.
In contrast, a throttle actuator of stepping motor type can produce a force which is large enough to control the intake air flow rate up to, for example, 8,000 rpm in terms of engine speed. Thus, the throttle actuator of stepping motor type can be used over wide range. It is also to be pointed out, however, that the magnet of the stepping motor, which is usually made of a ferrite alloy or an AL-Ni-Co alloy, exhibits a reduction in the magnetic power when its internal temperature exceeds a certain level. Thus, the torque of the stepping motor decreases in the condition through the point of contrary flexure where the magnet temperature exceeds about 120.degree. C. In such a case, since the stepping motor itself has no function for data concerning the rotational position of its rotor, the throttle opening is drastically decreased to restrict the flow of intake air to the same level as that in idle state of the engine. This phenomenon, usually referred to as "out-of-step", directly affects the running speed of the automobile and, hence, the safety of the passenger on the automobile, due to drastic closing of the throttle valve. From this point of view, the control of temperature of the throttle actuator motor is a very critical problem. Unfortunately, however, no proposals have been made for the control of the temperature of the stepping motor used in the throttle actuator.